Studies are focused on understanding the structure and function of the chlamydial polymorphic membrane protein D (PmpD) to better define its role in the biology of chlamydial infection and in its interaction with the host immune system. We have purified to homogeneity by immunoaffinity purification a native soluble fragment of PmpD that is secreted into the extracellular environment late in the infection cycle. The purified soluble fragment is being characterized at the protein level to define its secondary structural characteristics, and biologically by studying the interaction of the native protein with culture eukaryotic cells, human T cells, and dendritic cells. These studies are aimed at understanding the potential role of PmpD as virulence factor that might function in the suppression of chlamydial specific immune functions;specifically CD8 cytotoxic T cell immunity. PmpD is highly conserved antigenically among all C. trachomatis serovariants therefore a subunit PmpD vaccine based on the generation of a broadly cross-reactive protective antibody response is an attractive and ongoing goal of the laboratory. As the conformation of PmpD is essential for its ability to generate highly efficacious protective neutralizing antibodies we are using our protein structural analysis findings to design recombinant expression strategies capable of mimicking the proteins native structure. Although challenging, the success of these investigations are likely critical to the development of an efficious PmpD based vaccine. Using in vivo selection we isolated a hypervirulent C. trachomatis human urogential strain. The virulent isolate produces infection and disease in the mouse female genital tract with similar pathology to that of human infection. These findings are the first description of a human strain that is virulent for the mouse thereby providing a much needed small animal model for the study of human infection and disease. Remarkably, comparative genomic studies of virulent and avirulent clonal strains revealed a mutation in only a single gene unambiguously identifying the gene (CT153) as a critical in vivo virulence factor. These findings will enhance our understanding of the pathophysiology of human disease and better define at the cellular level immune mechanisms important to the devopment of protective immunity to infection. Future studies will focus on understanding how CT135 exacerbates infection and whether CT135 mutations are associated with chlamydial virulence in human disease. Defining the role of CT135 in pathogenesis could provide new insights important to chlamydial vaccine design and development.